This invention relates generally to imaging systems which may be used, for example, in connection with digital cameras, scanners, and the like.
Imaging sensors based on silicon technology typically use an infrared blocking element in the optical chain. The purpose of this infrared blocking element is to prevent infrared radiation (IR) or light (typically considered to be light with a wavelength longer than 780 nm) from entering the imaging array.
Silicon-based devices are typically sensitive to light with wavelengths up to approximately 1200 nm. If the IR is permitted to enter the array, the array responds to the IR, and generates an output image signal. Since one purpose of an imaging system is to create a representation of visible light, the IR introduces a false response and distorts the image produced by the imaging system. In a monochrome (black and white) imaging system, the result can be an obviously distorted rendition. For example, foliage and human skin tones may appear unusually light. In a color imaging system, the introduction of IR distorts the coloration and produces an image with incorrect color.
A common method for preventing IR based anomalies in imaging systems uses ionically colored glass or a thin-film optical coating on glass to create an optical element which passes visible light (typically from 380 nm to 780nm) and blocks the IR. This element can be placed in front of the lens system, located within the lens system, or it can be incorporated into the imaging system package. The principal disadvantages to this approach are cost and added system complexity. Thin film coatings can be implemented at somewhat lower cost, but suffer from the additional disadvantages of exhibiting a spectral shift as a function of angle. Thus, in an imaging system these elements do not provide a uniform transmittance characteristic from the center of the image field to the edge. Both filter types add to the system complexity by introducing an extra piece-part which must be assembled into the imaging system.
Digital imaging systems generally correct for what is called dark current. Dark current is what is detected by the imaging system when in fact no input image has been received. Generally dark current is isolated and subtracted either during a calibration process of the camera or on an ongoing basis. Mechanical shutters may be used to block off the optical system in between frames to provide a continuing indicia of dark current noise. This may be valuable because dark current is a strong function of temperature. Thus, it may be desirable to have a continuing indication of present dark current conditions. Dark current may also be continuously determined by providing certain pixels which are shielded from light to provide an indication of on-going dark current conditions.
Thus, there is a continuing need for imaging systems which reduce complexity and cost. In particular there is a need for a system which is sensitive to light in the visible spectrum and which is insensitive to light in the infrared spectrum, without requiring an infrared filter. Moreover, there is a need for a system which can continuously correct for the effects of both dark current and infrared noise.
In accordance with one embodiment, an imaging system includes a shutter that is selectively tunable in a first state to pass at radiation in the visible spectrum. In a second state, the shutter substantially blocks light in the visible spectrum while passing infrared radiation. A subtractor subtracts signals indicative of the radiation passed in the first and second states.